Insulating a suspended timber floor

Jonesy127

We have 1960s detached house which is very cold and draughty in the winter.

One of the main culprits is the suspended wooden ground floor, and I would really like to address the issue before this winter.

There is a crawl space under the floor, and 4 trap doors (under the stairs, kitchen, lounge and dining room) allowing entry; the space is just over 2ft high.

I assume the 2 most obvious techniques to insulate are either fit insulation roll between the joists held with wire/netting, or fit Celotex or similar between the joists?

I also intend to the seal the gaps between the skirting and floor, and insulate water pipes whilst I'm down there too.

Is there a big difference between the two, in terms of efficiency?

Would I be right assuming the insulation board route would be the more difficult and expensive option?

matelodave

I reckon the best way is with rockwool insulation held up with netting attached to the underside of the joists with a staple gun.

Celotex & Kingspan is very expensive and you'd struggle to get it cut and fitted into place unless you've got a small child to wriggle in and out with it all. I for one would never manage to get into a 2' crawl space.

You can work out how efficient they would be by doing the sums using the u or r values of the insulation you are considering - you'd need to compare the differing thickness's of Celotex with say 100 or 200mm of rockwool.

I did contemplate insulating my ground floor but as I'm too big to negotiate the space under the joists the only way it could be done was to pull up all the floor boards and then relay or replace them.
I came to the conclusion that the cost & effort would be significantly greater than the benefit.

Do the draught proofing and then put down carpet with underlay.

Richie-from-the-Boro

I'd use sheeps wool, but there's only about a quid a square meter between it and Celotex or similar, but you only do it once :

- sheep 0.16 U-value can be achieved with a depth of 225mm [8¾in] thick
- Celotex, and some other brand names, 0.16 U-value 175mm [6¾in] thick

The thing is working from below unless you are very lucky and the joists are likely to be evenly spaced its going to be a megga problem to cut those batons cut accurately enough to work the U value and stop drafts. If you choose to use Celotex or whatever cut them all short by 1/4" to make your life easy then filling the gaps with expanding foam kills two birds with one stone. Alternatively you can taper the cut very slightly so the wedge in of their own accord but if you do this make sure the baton is right up to the top of the joists.

Insulating under ground level floorboards will do two unexpected things that must be taken into account. It would be expected when built that the house would breathe out condensation through the floorboards so (1) watch out for covering airbricks and if you thought it was cold under there it will be (2) three times colder after you have insulated. You have already realised that filling the board gaps through which air can flow is critically important, not doing so will undo the work of the insulation. A breather membrane under the insulation and taped to cover any holes will both hold the insulation in place and stop any draughts.

There's a whole other area of draughts and cold about an inch deep all around your room called a skirting board also needs to be considered, ditto central heating pipes that pop up through the floors etc, and if you have a fire with an open chimney and do not 'plug it' when not in use most of the above is a waste of money and effort. In which case hardboard, a good dose of insulating underlay and carpet is your best bet.

Best of luck !

Ken68

Same situation here Joneey,I insulated 10 year ago with Celotex, very awkward, good end result but netting and Rockwool would be easier and certainly cheaper.
Chance also to clean out and tighten the floorboards.